 Welcome to real gases. Today we're going to compare the van der Waals equation with the ideal gas law. The ideal gas law is used to describe the relationship among the pressure, volume and temperature of a gas sample containing n moles of gas. It's an approximation because it assumes that gas molecules have no volume and do not attract one another. At ordinary temperature and pressure, where the ideal gas law is a good approximation, the small finite volume of real gas molecules is nearly negligible compared to the volume of empty space. However, the volume of molecules becomes significant as the pressure is increased, forcing the molecules closer together and reducing the volume of empty space available to each molecule. The van der Waals equation describes a real gas by reducing the volume with a correction factor. The volume term of the ideal gas law is replaced by v minus nb, where n is the number of moles of gas and b is a constant that varies for each gas. The ideal gas law assumes that the gas molecules do not attract one another. As the pressure on a gas sample is increased and the molecules become closer together, there is an attraction between molecules. The attraction of the gas molecules to each other has the net effect of reducing the force of collisions of the gas molecules with the walls. This reduces the pressure felt at the walls. To take this effect into account, the van der Waals equation adds a factor to account for the reduction in the pressure term. The pressure term of the ideal gas law is replaced by p plus n2a divided by v2, where n is the number of moles of gas, v is the volume of the gas and a is a constant for a given gas. With these corrections to the ideal gas law, the van der Waals equation is shown here. Although this equation will predict the behavior of real gases more accurately, the ideal gas law works well when pressures are close to or lower than one atmosphere. At these pressures, real gases act like an ideal gas because the gas particles are far apart. Let's test your knowledge. Real gases behave differently than an ideal gas, which one of the following statements are true about a real gas. Real gases have finite volume and attract each other. You've completed real gases.